174 NOTES
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Inorgunic Chemistry
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Figure 2.-Chemical shift of the OH proton resonance as. temperature for a 1 mol solution of dimethylboric acid in methylcyclohexane. Shifts relative t o cyclohexane are calculated by subtracting 1.44ppm from the chemical shift relative to tetramethylsilane (TMS).
purities, such as methylboric anhydride, than is diinethylboric acid from methylboric acid. The purification of dimethylboron chloride is also tedious4 The broad OH stretching band in carbon tetrachloride and the concentration and temperature dependence of the hydroxyl proton chemical shift are typical for hydrogen-bonded protons. S o attempt has been made to calculate the bond energy from the temperature dependence of the nmr spectrum, since the system involves several species in unknown concentrations. Further, extrapolation to obtain the maximum downfield shift of the hydrogen-bonded proton is somewhat uncertain (see Figure 2). It is of interest to examine the present resuIts in terms of a correlation proposed by Ferraro and Peppard ;8 these authors found a linear relation between the chemical shift of the hydroxyl proton in the region of "minor bond breaking" ( i e . , the relatively flat portion of Figure 1 a t higher concentration) and the hydrogen-bond energy. A similar relation was noted between the bond energy and the difference in the infrared stretching frequencies of free and hydrogenbonded OH groups.* Using this correlation, our value of ca. 7.3 ppm downfield from cyclohexane (8.7 ppm from tetramethylsilane) for the chemical shift of the hydroxyl proton in the region of minor bond breaking corresponds to a hydrogen-bond energy of 4.6 kcal. Furthermore, the value of 260 cm-I obtained by subtracting the frequency of the bonded OH from the frequency of the free OH corresponds to an energy of 4.8 kcal. The good agreement between ( 8 ) J. 12. Ferraro and U. F. Pegpard, J . Phrs. Chem., 67, 2639 (1963).
these two values leads us to place some confidence in the values obtained. It is of interest to note that the shift of the hydroxyl proton of the monomer, cu. 4.3 ppm downfield from tetramethylsilane, occurs a t a much lower field than that of the monomeric proton in ethanol, 0.76 ppm downfield. This would indicate decreased shielding of the hydroxyl proton in dimethylboric acid, which is in keeping with the expected lowering of the electron density on the oxygen atom caused by the acceptor capability of the boron atom. Oxygen-boron backdonation would also account for the failure to observe any indication of intermolecular coordination ; this is not unexpected, however, since methoxydimethylboron is not associatedIO despite the electron-releasing methyl group on oxygen. Evidence for oxygen-boron T bonding, in terms of hindered rotation about the boron-oxygen bond, has recently been provided;" the splitting of the B-methyl resonance in methoxydimethylboron a t - 44' Iyas attributed to this effect. Splitting of the methylboron resonance in dimethylboric acid was not observed down to - loo", presumably because very rapid hydrogenbond breaking and forming averages out the environments of the B-methyl groups, irrespective of hindered rotation about the boron bond. Acknowledgment.--This research was supported in part by the Defense Research Board of Canada, Grant No. 9530-49. The authors wish to thank Dr. M. G . Hogben for the A-GO nmr spectra. (9) This value is derived from t h e work of E . 1). Becker, L , Liddel, and J. N. Shoolery, J . Mot. S p e c t r y . , 2, 1 (1958), on t h e assumption t h a t t h e central peak of t h e CHs triplet is 8.83 ppm downfield from tetramethylsilane. (10) G. E. Coates, J . Chem. Soc., 3481 (1950); A. B. Burg and I
